Retardation films have functions to convert linearly polarized light into circularly polarized light or elliptically polarized light and to change the polarization axis of linearly polarized light in a certain direction to another direction (optical rotation). By using these functions, it is possible to improve, for example, visible angle and contrast of liquid crystal display devices. The retardation film can be obtained by uniaxially or biaxially stretching plastic films such as polycarbonate, polyarylate and polyether sulfone. In this instance, birefringence is generated due to an anisotropy in refractive index generated by stretching, and hence the film works as a retardation film. Optical performance of a retardation film can be determined by retardation value which is calculated from a difference between a refractive index in the slow axis direction (a direction in which refractive index in the plane becomes the largest) and a refractive index in the fast axis direction (a direction orthogonal to the slow axis direction in the plane) along the front direction of the retardation film at a certain wavelength multiplied by a thickness of the retardation film. However, the retardation value exhibits so-called wavelength dependency (wavelength dispersion characteristics) and visible angle dependency (visible angle characteristics), and the retardation film is used for various types of display devices considering comprehensive performances including these various characteristics.
The wavelength distribution characteristics differ depending on a type of material to be used, and the retardation film made of general polymer materials have the characteristics that the retardation value in the wavelength side longer than 550 nm is smaller than the retardation value at a wavelength of 550 nm and the retardation value in the wavelength side shorter than 550 nm is larger than the retardation value at a wavelength of 550 nm.
Accordingly, even if a retardation value at a wavelength of 550 nm is adjusted for 137.5 nm to produce a quarter retardation film, the retardation in the wavelength side longer than 550 nm becomes ¼ or less of the wavelength, and the retardation in the wavelength side shorter than 550 nm becomes ¼ or more of the wavelength.
This poses, for example, the problem that when a retardation film (so-called quarter wavelength plate) having a retardation which is ¼ of the wavelength is used to produce an anti-reflection filter, a sufficient anti-reflection effect is obtained only in the wavelength range where the retardation is almost ¼ of the wavelength and circularly polarized light is converted into elliptically polarized light at other wavelengths resulting in that only insufficient anti-reflection effect is obtained. Also, when a retardation film (so-called half wavelength plate) having a retardation which is ½ of the wavelength is used to produce a rotary polarizer which is used for a liquid crystal projector and the like, only in the wavelength range where the retardation is almost ½ of the wavelength can rotate a linearly polarized light as linearly polarized light, and linearly polarized light is converted into elliptically polarized light at other wavelength with the result that only an insufficient rotary polarizing effect is obtained.
A retardation film having such wavelength distribution characteristics that impart the same level of retardation to a wavelength through the whole range of visible region is called as an achromatic retardation film, and it is necessary to exhibit such tendency (achromatic property) that, for example, the retardation value in the wavelength side longer than 550 nm is larger than the retardation value at a wavelength of 550 nm and the retardation value in the wavelength side shorter than 550 nm is smaller than the retardation value at a wavelength of 550 nm. As a method to produce such a retardation film, for example, a method has been proposed in which a plurality of stretched films are laminated with their optical axes being crossed over with each other, as described in Patent Reference 1. Further, a retardation film has been proposed, which is produced by using cellulose acetate obtained by hydrolyzing cellulose triacetate and can impart the same level of retardation to each wavelength in a wide range of the visible region by using only one film, as described in Patent Reference 2.
Also, a technique to produce a retardation film by using liquid crystal compounds which can be oriented on a substrate which has been subjected to rubbing treatment and a composition containing a plurality of such compounds, and orientating said compounds in a specific direction has been known, as described in Patent Reference 3. Further, a retardation film obtained by orientating a liquid crystal compound in a specified direction has wavelength distribution characteristics that can impart the similar level of retardation to each wavelength in a wide range of the visible region by using only one film, as described in Patent Reference 4.
On the other hand, visible angle characteristics can be generally controlled by a stretching method of the retardation film. In the case of usual retardation film obtained by uniaxially stretching a polymer film such as polycarbonate, when the film is tilted in the slow axis direction (a direction in which refractive index becomes the largest in the film plate), retardation value becomes smaller as the tilt angle from the front direction becomes larger, and contrary, when the film is tilted in the fast axis direction (a direction orthogonal to the slow axis in the film plane), retardation value becomes larger as the tilt angle from the front direction becomes larger. This tendency is one commonly observed with other usual uniaxially stretched retardation films of polyarylate, polyether sulfone and cycloolefin polymer such as Zeonor (trade name, made by Zeon Corp.) and Arton (trade name, made by JSR). If retardation value changes when the film is tilted as described above, visible angle characteristics become worse. Therefore, it is demanded that retardation value does not change by tilting in an application where visible angle characteristics become a subject of discussion. When retardation value does not change at all by tilting, retardation value shows little change even if the film is tilted in any direction of the slow axis direction and the fast axis direction, and such retardation film is demanded.
To solve this problem, a retardation film has been disclosed in Patent Reference 5, in which change of the retardation value by tilting is controlled by laminating a film to be stretched and a shrinkable film, and practically biaxially stretching by uniaxially stretching the laminated film.
Patent Reference 1: JP 3174367
Patent Reference 2: JP-A-2000-137116
Patent Reference 3: JP-A-2000-98133
Patent Reference 4: WO 02/093213A1
Patent Reference 5: JP 2818983